The oligo- or polymerization of isocyanates, especially to form higher molecular weight oligomer mixtures having uretdione (“dimer”), isocyanurate (“trimer”) and/or iminooxadiazinedione structures (“asymmetric trimer”) in the molecular skeleton, collectively called isocyanate modification here, has long been known. If the modified polyisocyanates contain free NCO groups, which optionally may also have been temporarily deactivated with blocking agents, they are exceptionally high-quality starting materials for the production of a multitude of polyurethane plastics and coating compositions.
In a series of industrial methods for isocyanate modification which have become established, the isocyanate to be modified, usually a diisocyanate, is generally converted by addition of catalysts and these are then rendered inactive (deactivated) by suitable measures when the desired degree of conversion of the isocyanate to be modified has been attained, and the polyisocyanate obtained is generally separated from unreacted monomer. A summary of these methods from the prior art can be found in H. J. Laas et al., J. Prakt. Chem. 1994, 336, 185 ff.
Useful modification catalysts have been found to be neutral bases and compounds of ionic composition. The latter can usually be used in a very small amount and lead extremely rapidly to the desired result. In the case of the neutral bases, depending on the monomer to be converted and the neutral base used, this is not always true, but it is virtually impossible to infer structure-effect or -activity relationships (cf. Chem. Eur. J. 2009, 15, 5200-5202).
The option of also using tetraorganylammonium or -phosphonium as cation to the anion which is catalytically active toward isocyanates, such as hydroxide, alkanoate, alkoxylate, etc., is common knowledge, although generally not explicitly emphasized as being particularly preferred; cf. H. J. Laas et al., J. Prakt. Chem. 1994, 336, 185 ff.
Additionally known is the use of fluorides and hydrogenpolyfluorides, the latter being stable adducts of HF with compounds containing fluoride ions, optionally also in the form of their ammonium or phosphonium salts, for the isocyanate modification, from documents including EP 962 455 A1, EP 962 454 A1, EP 896 009 A1, EP 798 299 A1, EP 447 074 A1, EP 379 914 A1, EP 339 396 A1, EP 315 692 A1, EP 295 926 A1 and EP 235 388 A1.
However, the tetraorganylammonium and -phosphonium (hydrogenpoly)fluorides of the prior art, in the performance of the modification reaction, often have the disadvantage that, when they are used, the reaction can sometimes be maintained only with continuous metered addition of catalyst, meaning that the breakdown of the catalyst in the isocyanate medium proceeds unacceptably quickly for technical purposes compared to the modification reaction.
This disadvantage is not even always eliminated in a satisfactory manner by addition of aminosilanes to substances including fluoride ions, as described in EP 1 318 160 A1. It is possible that the compounds described as fluoride source therein are inactive without addition of aminosilanes or do not have sufficient activity for the isocyanate trimerization.
U.S. Pat. No. 5,260,436 B discloses the catalyzed reaction of aromatic isocyanates with specific bicyclic bases having P—N bonds (called phosphatranes) to give isocyanurates. However, J. Org. Chem. 2010, 75, 5308-5311 explicitly points out that the corresponding reaction with aliphatic isocyanates does not take place.
According to the teaching of CN 102964566, asymmetric trimer-containing products are obtainable, although the activity of the catalysts is generally unacceptably low for industrial implementation of the process, and catalyst breakdown occurs extremely rapidly with formation of carcinogenic phosphoramides, e.g. HMP. This is demonstrated in comparative example 5 of the present application. Furthermore, the process products of CN 102964566—in contrast with statements to the contrary in the cited document—are highly contaminated with troublesome by-products, especially with uretonimines.
EP 2 415 795 A1 describes very stable tetraorganylphosphonium (hydrogenpoly)fluorides that do not have these disadvantages, but they are not commercially available and are preparable only with difficulty.